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RNA-mediated virus resistance is one kind of post-transcriptional gene silencing (PTGS). PTGS can be induced by transgenes or by viral infection that triggers the degradation of RNAs with homology or complementarity to the foreign genes’ transcript or viral RNA. In this study, tobacco plants transformed with untranslatable Potato virus YN (PVYN)-coated protein (CP) gene were used to examine the correlation between transgene integration patterns and RNA-mediated resistance more directly. HindIII and EcoRI were used to digest genomic DNAs from progeny T1 transgenic plants with a range of responses to PVYN infection. Southern blot analysis indicated that nearly all of the highly resistant plants contained repeat foreign sequences, including direct repeat (DR) and inverted repeat (IR). Specifically, the highly resistant plants always contained both or one of these constructs, with the majority containing both constructs. Conversely, almost all of the susceptible transgenic plants contained neither construct, though some contained multiply copies. Our results show that transgene integration rather than copy number is the key factor for RNA-mediated virus resistance. Many researchers identified double-stranded RNA (dsRNA) as the common link for PTGS triggered by various events. Therefore, we suggest that only transgenes can make dsRNA, by transcription or other pathways, and either repeated constructs (IR or DR) or single copy will induce RNA-mediated PVYN resistance. [ABSTRACT FROM AUTHOR]

The complete genomic sequence of a Chinese Potato virus X isolate FX21 (PVX-FX21) was determined from three overlapping cDNA clones. The genome of PVX-FX21 is 6435 nucleotides in length excluding the poly(A) tail and contains five open reading frames (ORFs). Its entire genomic sequence shares 95.2–96.3% identities with Asian and European isolates, and 77.3–77.8% with American isolates. Phylogenetic analysis of the complete genomic sequence reveals two groups: the Eurasian group and the American group. PVX-FX21 belongs to the Eurasian group and forms a separate sub-branch with three Asian isolates. Similar analyses of the coat protein genes of 37 PVX isolates also reveal two major groups. All PVX isolates from Asia are clustered to group I, whereas isolates from Europe and America are clustered to both groups. Nucleotide sequence diversity analyses show that there is no geographical differentiation between PVX isolates and that constraint on the ORF encoding RNA-dependent RNA polymerase is much higher than those on the other four ORFs. [ABSTRACT FROM AUTHOR]

Tobacco vein banding mosaic virus (TVBMV) is of increasing importance in tobacco production on the Chinese mainland. The 3′-terminal genomic sequences (1624 nucleotides) of 12 TVBMV isolates from China were determined and compared to the sequences of only four TVBMV isolates available in databanks. The results revealed that TVBMV consists of several phylogenetically distinguishable strains that show a degree of correlation with the geographical origin. Two isolates from Yunnan had a unique putative NIb/CP proteolytic cleavage site of Q/N that is uncommon for potyviruses, whereas other TVBMV isolates had the more typical Q/G amino acids at that site. One isolate (ZB6) from Zibo, Shandong Province, was predicted to have experienced recombination within the characterized genomic region. [ABSTRACT FROM AUTHOR]